Fossilized sediment from New Jersey's salt marshes contains evidence of a migrating coast line. For some 2,000 years, up until the dawn of our modern warming era around 1900, the sea level off of what's now New Jersey was rising by about one to two millimeters a year, with the coast itself imperceptibly creeping inland. Today, the sea level is rising by three to five millimeters a year.
Perhaps that still doesn't sound like much. But this is the point of taking a very long view of history.
"The last time we saw rates as fast as this was 6,000 years ago," says Benjamin Horton, an associate professor in the Department of Earth and Environmental Science at the University of Pennsylvania.
And what was happening 6,000 years ago? Temperatures were rising then, too, although they weren't as warm as they are today.
"Because we came out of a glacial period, oceans were warming, and ice sheets were melting," Horton says. "They're exactly the same processes that we're seeing today."
Horton and fellow researchers have used that fossilized evidence to reconstruct sea-level rise around New Jersey going back 10,000 years in research newly published in the Journal of Quaternary Science. To do this, they collected sediment cores drilled tens of meters below ground from coastal marshes, then examined the sediment back in a lab for microscopic organisms that only exist at specific depths below sea level. Salt marsh grasses also fossilized within the sediment were used to radiocarbon-date the samples.
The 10 maps contained in the GIF below show the movement of sea level at 1,000-year intervals leading up today:
Topography in meters. GIF built with maps courtesy of Benjamin Horton.
The black outline represents the coast as it exists today. The "0" on the color bar above is sea level, with green, yellow, orange, and red areas showing elevation in meters above sea level, and teal and blue showing the depth of the ocean below it. For reference, this is the same land on a map you may more easily recognize, from Chesapeake, Virginia to Boston:
For coastal cities still recovering from Superstorm Sandy, this history should be particularly alarming.
"If you stood at Atlantic City on the boardwalk, at that same spot 10,000 years ago, the sea level would be 40 meters lower," Horton says. "You just have to think how far out it would be. It is thousands of meters beyond its present day location."
Communities along the New Jersey coast are already trying to restore beaches even as they slowly subside, hauling in sand from elsewhere. Horton suggests that that task – and the cost of executing it – will only grow bigger from now on based on what we now know about the historic trajectory of sea-level rise.
"The problem that we have is that people are unwilling to accept climate change, and we should just accept it," he says. "We have the ability now to start to think about what the rates of rise in the future will be."